Telomeres are specialized structures that "cap" the ends of eucaryotic chromosomes, thereby allowing chromosomes to exist as stable linear DNA molecules. The loss of function of a single telomere is sufficient to activate cell cycle checkpoints and arrest cell division. However, if cells with compromised telomere function evade checkpoint surveillance and continue to proliferate, they gradually lose the protective function afforded by telomeres, resulting in chromosome end-to-end fusions and illegitimate recombination events. Thus, loss of telomere function can contribute to genetic alterations that promote tumorigenesis. Our long-term objective is to understand the processes critical for maintaining the chromosome capping function of telomeres throughout the cell cycle, and this proposal is focused on examining the role of three proteins, Cdcl3p, Stn1p, and Ten1p, in maintaining telomere integrity in the budding yeast S. cerevisiae. Cdc13 binds to single-stranded telomeric DNA sequences in vitro and in vivo, and there is evidence to suggest that Cdc13, Stn1 and Ten1 physically interact as part of a protein complex. The activity of this putative complex is essential for telomere function since mutant forms of Cdc13, Stn1 and Ten1 all lead to the accumulation of single- stranded DNA at telomeric regions and result in defective telomere length regulation. In the work proposed here we will: A) Test whether Cdcl3p, Stn1p and Ten1p function as a single protein complex and determine the functional significance of these interactions; B) Test the hypothesis the accumulation of single-stranded DNA in cdc13, stn1 and ten1 mutants reflects a requirement for Cdcl3p, Stn1p and Ten1p in telomere DNA replication; and C) Determine the types of chromosome aberrations that occur in strains deficient for telomere integrity and utilize the genetic strengths of the yeast system to identify new factors critical for maintaining telomere capping function. This work will not only contribute to elucidating the role of Cdc13p, Stn1p and Ten1p in telomere function but will more generally expand our understanding of the mechanisms that maintain chromosome end protection throughout the cell cycle.